The present invention relates to a method for detecting fingerprints deposited on various substrate bodies using a laser and an apparatus therefor useful in the field of criminal identification.
As is known, various methods are used in the prior art for the detection of a latent fingerprint including the solid methods, in which a powder of aluminum and the like is sprinkled over the samples and deposited on the moisture and fat in the secretion deposited on the sample so as to produce visually detectable difference in the color tone between the background surface and the fingerprint pattern, liquid methods, in which the amino acids and salinity contained in the deposited secretion are subjected to a color reaction with a chemical reagent, and gas methods, in which the fat contained in the deposited secretion is reacted with iodine gas to cause coloration.
These prior art methods, however, are not free from several disadvantages and defects that the detecting power thereof is low when the fingerprint is aged after deposition on the sample or when the fingerprint is deposited on the surface of a certain kind of samples such as non-traditional building materials coated with a surface film of a polymer and that the sample is sometimes stained or contaminated unduly. As a remedy for these defects, an improved method has been proposed by utilizing the fluorescence produced by the irradiation with a laser beam in several publications including:
(1) E. R. Menzel, Identification News, International Association for Identification, volume XXXIII, No. 9 (September, 1983); and
(2) R. D. Olsen, Identification News, International Association for Identification, volume XXXIV, No. 4 (April, 1984).
The so-called laser fluorescence methods hitherto reported are classified into three classes of:
(a) a method in which the fingerprint as deposited is irradiated as such with a laser beam and the fluorescence emitted from an ingredient, e.g. riboflavin, in the fingerprint is utilized for detection;
(b) a method in which certain constituents, e.g. amino acids, urea and the like, in the secretion are reacted with a chemical reagent to be converted into a fluorescent material before irradiation with a laser beam; and
(c) a method in which the sample bearing a latent fingerprint deposited thereon is first contacted with a fluorescent dye before irradiation with a laser beam.
The above mentioned method (a) is, however, hardly applicable to a sample emitting a relatively strong background fluorescence to mask the generally weak fluorescence from the fingerprint pattern since a fingerprint usually contains only an extremely small amount of the fluorescence-emitting constituents other than water although the method is advantageous in respect of the absence of contamination on the surface of the sample.
The above mentioned methods (b) and (c) are more effective than the method (a) when the sample bearing the fingerprint emits a background fluorescence but these methods are not free some problems as given below in respect of the procedure of dipping the sample in the reagent solution or coating or spraying a solution of the reagent or dye on to the sample.
The above described prior art publications (1) and (2) describe a method of dipping the sample in the reagent solution or spraying the reagent solution to the sample, of which following disadvantages are unavoidable.
(i) When a sample bearing a latent fingerprint deposited thereon is dipped in a dye solution, it is usual that the solution adheres to the surface in a more than sufficient amount. Accordingly, the fluorescence may be emitted not only from the lines of the fingerprint pattern but also from the background surface almost as strongly as from the fingerprint lines per se so that difficulties are encountered in detecting and identifying the image of the fingerprint.
(ii) It is essential in practicing the method to strictly control the conditions of dipping or spraying. For example, the length of time for dipping in the case of the dipping procedure and the volume of sprayed solution in the case of spraying procedure must always be kept constant making the method very troublesome or time-consuming. when an excessively large volume of the reagent solution is taken up on the surface of the sample, in addition, the ingredients in the fingerprint secretion or reaction products thereof on the sample may sometimes be dissolved or eluted out in the solution so that distinctness of the fluorescent fingerprint image is decreased as a consequence.
(iii) The method is poorly versatile because the method is practiced using a solution of the reagent or dye in the actual spot of, for example, a criminal case. In addition, the sample is sometimes unduly contaminated with the solution. Further, the spraying procedure naturally requires a sprayer which is not always convenient in handling even by setting aside the disadvantage due to the cost therefor.
The method (c), which is advantageous in the presence of a relatively strong background emission, has problems of fluorescence emission from the dye extraneously deposited on the portions outside the fingerprint lines and indistinctness of the fingerprint lines due to the dissolution of the fingerprint secretion in the solvent of the dye solution. Further, the surface of the sample is unduly contaminated with the dye solution and this disadvantage is particularly serious when the sample is made of a material readily soaked with the solution such as paper or cloth.
The publication (1) for the method (c) describes particular examples using rhodamine 6G and ninhydrin as the typical dyes.
According to the disclosure for the procedure using rhodamine 6G, a methyl alcohol solution of the dye is applied or sprayed to the sample followed by drying and the sample is irradiated with the laser beam. It is taught here that an excessive amount of rhodamine 6G deposited on the surface should be removed by washing with methyl alcohol.
The applicablility of this method to practical cases is, however, questionable in respect of the detecting power of fingerprints due to the following difficulties. When methyl alcohol is used as the solvent, the wettability of the surface of samples is usually much larger to methyl alcohol than to water and the dye solution is dried up as such so that the dye is deposited and adheres in a considerably large amount to the surface outside the fingerprint lines to cause difficulties in distinguishing the fluorescence from the fingerprint lines alone. Methyl alcohol in itself is not suitable as the solvent used in this purpose because methyl alcohol has dissolving power of the fatty ingredients in the fingerprint secretion and dehydrating power to cause loss of a large part of the fingerprint ingredients. In addition, some of the materials forming the samples are susceptible to the attack of methyl alcohol. When an excessively deposited dye is removed by washing with methyl alcohol, the dye once fully dried and firmly adhering to the surface can hardly be washed away without the danger of losing a large part of the fingerprint ingredients.
An alternative method belonging to the method (c) utilizes a chemical reaction of ninhydrin typically applicable to the detecting purpose of fingerprints deposited on a sample readily soaked with a liquid.
In a practical procedure using ninhydrin, the sample is coated with ninhydrin followed by drying to produce a contrast in color between the background and the fingerprint deposition by which the fingerprint can be detected. A problem in this method is the inapplicability of the method to certain samples such as thermosensitive paper and the like because blackening takes place all over the areas coated with ninhydrin rendering detection of the fingerprint lines impossible.
Various types of apparatuses for the detection of fingerprints using a laser are commercially available as manufactured by American companies including (3) Spectra Physics Co., (4) Laser Ionics Co., (5) Plasma Kinetics Co. and (6) Laser Photonics Co. according to their catalogues.
A major current of the fingerprint detecting methods using a laser utilizes the principle according to which a fluorescent substance deposited on the fingerprint lines by a suitable method is excited by the irradiation with a laser beam to emit fluorescence detectable as a fingerprint image. The laser beam is usually green in color so that the a fluorescence emission is obtained in the longer wavelength region ranging from yellow to orange. The prior art technology in connection with the detecting instrument can be classified into two classes as described below.
(1) The instrument is constructed either of three parts including a laser unit composed of a laser oscillator and a power supply therefor, an image-receiving unit composed of a sharp-cut filter and a two-dimensional image receiver and an image monitoring unit or of the laser unit alone leaving the procedure of the fingerprint detection to visual inspection through an optical filter. The laser used here is a large argon ion laser or a copper vapor laser.
The laser unit in the instruments of this type is usually large and heavy and requires supply of cooling water and large electric current with poor portability and mobility as a serious disadvantage. Accordingly, most of the conventional instruments of this type must be installed in a station for the works of fingerprint detection and identification.
(2) On the other hand, the fingerprint detector instrument manufactured by Laser Photonics Co. is portable with a small YAG laser although the instrument is constructed with the units of the same type as in the instruments described in (1) above. In this instrument, the laser beam is conducted to the sample bearing a latent fingerprint by means of an optical fiber to excite the fluorescence emission from the fingerprint lines and the image of the fluorescent fingerprint is taken by a separate TV camera. Further, a super-high sensitivity image receiver is used as the two-dimensional image receiver in this instrument so that the image receiving unit cannot be compact with additional disadvantages in respect of the S/N ratio and resolution of the fingerprint images.
Though advantageous in respect of the absence of necessity of moving the laser oscillator, the instrument of this type is disadvantageous because the operability of the instrument is poor as a result of the separate installation of the units for the laser beam irradiation and image receiving. Moreover, the instrument cannot be operated by a single operator.
In addition, each of the above described instruments (1) and (2) cannot be a single integral unit because of the dimensions and weight of each of the component units to decrease the operability of the instrument. Further, there may be some danger when the diffused reflection of the laser beam from the sample hits the operator's eyes. This problem is particularly serious when the laser is a high output one.